1. Field of the Invention
The present invention relates to a process for producing a steel sheet or strip for making a can obtained by drawing and ironing, of the beverage can type.
The present invention also relates to a steel sheet or strip for making a can obtained by drawing and ironing.
2. Description of the Background
This type of can usually comprises a bottom, a thin peripheral wall and a neck for achieving the setting or seaming of a lid, which may be of the easily-opened type, and is produced in particular by drawing and ironing a cup cut from a metal sheet or strip.
For this purpose, the cup is subjected first of all to a drawing operation with a relatively severe reduction on a press which comprises in the conventional manner, on one hand, a fixed punch and a support forming a peripheral blank holder which is slidable around said punch and on which the cup rests, and, on the other hand, a die to be applied against the cup with a force transmitted vertically by an upper slide.
The cup, comprising a bottom and a flange formed by the drawing operation, is then either calibrated by a light drawing operation without the use of a blank holder, or redrawn with a blank holder and is then subjected to an ironing operation which comprises drawing the flange, by means of a draw die with successive reductions, so as to progressively form the thin peripheral wall of the can.
Thereafter, the bottom is formed on the draw die so as to impart thereto a given geometry and the neck of the thin peripheral wall is formed in accordance with two methods, namely a necking method with a die, named die-necking, or a necking method employing a forming roller, named spin necking
The method for necking with a die comprises forcing the neck into a die having a conical inlet profile and a cylindrical outlet profile. A cylindrical element guides the formed wall as it leaves the die.
The force required to permit the deformation of the metal is derived from the thrust applied on the bottom of the can and axially transmitted by its thin peripheral wall.
To reach the desired inside diameter, a plurality of successive reductions are required, each being a distinct forming step. When the reduction in diameter is obtained, flanging is usually effected with flanging rollers.
The spin-necking method employing a forming roller comprises driving the can in rotation while it is maintained between a pusher and a centring ring.
The free end of the thin peripheral wall is engaged on a mandrel and two axially travelling rollers form the neck of the can which progressively leaves the mandrel while always being maintained between the pusher and the centring ring.
The profile of the neck is obtained by the simultaneous displacements of the rollers, the centring ring and the pusher.
Subsequent to these various operations, the can is filled and a lid, for example of the easily-opened type, is set or seamed on the neck of the can.
It is known to use for making this type of can a steel sheet or strip of extra-soft steel type the composition of which in percentage by weight is the following:
Carbon of the order of 0.030 to 0.040% PA1 Manganese of the order of 0.15 to 0.25% PA1 Nitrogen of the order of 0.004 to 0.006% PA1 Aluminium of the order of 0.03 to 0.05% PA1 Phosphorus less than 0.015% PA1 Sulphur less than 0.020% PA1 Silicon less than 0.020%, PA1 a maximum of 0.08% of one or more elements selected from copper, nickel and chromium, the remainder being iron and residual impurities. PA1 Carbon less than 0.008% PA1 Manganese between 0.10 and 0.30% PA1 Nitrogen less than 0.006% PA1 Aluminium between 0.01 and 0.06% PA1 Phosphorus less than 0.015% PA1 Sulphur less than 0.020% PA1 Silicon less than 0.020% PA1 a maximum of 0.08% of one or more of the elements selected from copper, nickel and chromium, the remainder being iron and residual impurities, in which process the slab is hot rolled into a hot sheet or band having a thickness of less than 3 mm, then the hot sheet or the band is cold rolled with a reduction of between 83 and 92% and subjected to a recrystallization annealing at a temperature lower than Ac1 and finally cold rerolled with a reduction of between 10 and 40%.
The sheet or the strip is produced by a process in which the slab issuing from a continuous casting operation is hot rolled, then cold rolled to obtain a thin sheet or foil which is subjected to a recrystallization annealing operation at a temperature below Ac1.
This process permits obtaining a thin sheet or foil which has a final thickness of about 0.30 mm, and making from this sheet a can whose thin peripheral wall has, after drawing and ironing, a thickness of the order of 0.1 mm.
Now, manufacturers of cans, for reasons of economy and increased productivity, aim at producing cans of reduced weight, i.e. with thinner walls.
In order to enable the can with thinner walls to withstand the pressure of the liquids it contains, particularly when it concerns a gaseous beverage, and in order to ensure that the can itself has a sufficient strength, steels of improved mechanical characteristics must be used.
In order to improve the mechanical characteristics, manufacturing firms have, with the use of an extra-soft steel of the aforementioned composition, subjected a slab to a hot rolling and to a cold rolling to obtain a sheet which is subjected to a recrystallization annealing at a temperature below Ac1, and is then cold rerolled.
But it is known that a reduction in the thickness or an improvement in the mechanical characteristics of sheets or strips accentuates the phenomena of creasing when making the cans.
Tests have shown that this process results in a narrowing of the range of drawability of the sheet or the strip and an increase in the earing coefficient.
A narrower drawability range results in difficulties in the forming of the bottom and is the origin of the appearance of wrinkles during the drawing operation.
In order to avoid the formation of wrinkles when drawing, the pressure exerted by the blank holder on the sheet blank may be increased, but this increase in the pressure of the blank holder creates a problem of the control of the flow of the metal during the drawing and may consequently cause the metal to fracture or tear, particularly in the region of the connection or corner radii.
Further, the increase in the earing coefficient creates a problem when removing the can from the draw punch, i.e. during the stripping operation.
Indeed, this operation is carried out by sliding a ring along the draw punch so that it can bear against the free edge of the thin peripheral wall of the can.
When the thin peripheral wall of the body of the can has large earings, the stripping ring only bears against a few points of the peripheral wall and very often there occurs a creasing of the peripheral wall during the stripping and the can must be scrapped.
In order to reduce the earing coefficient, it is known to coil up the sheet in the hot state before the cold rolling and recrystallization annealing.
But this additional operation results in drawbacks since the edges of the sheet or strip are in direct contact with the surrounding air and cool quicker than the centre part.
This natural cooling differential between the edges and the centre results in a heterogeneity of the mechanical characteristics of the sheet or strip. Moreover, the coiling up of the sheet in the hot state results in the formation of a coarse cementite.
The coarse cementite may result in the piercing of the thin peripheral wall when forming the neck and a tearing away of this metal during the drawing operation owing to hard particles in the steel.
Further, the presence of hard particles in the steel results in a premature wear of the various drawing and ironing tools.
Consequently, manufacturers are faced with serious problems which are often antinomic when they attempt to reduce the thickness of the walls of the cans.